Programmable clocks are useful, for example, in schools where class schedules remain relatively fixed and bells or other audible devices are used to signal the beginning and end of each class period. The timing and control of the bells or other audible devices can become relatively complex because different zones of the school can be on different schedules, the schedule can change from day to day, special events such as holidays or assembly days can dictate a changed schedule for a particular day, and the like. In addition, it is very desirable for the system to provide some simple means for altering the timed events, such as when schedules change, or with the time change to or from daylight savings time.
In the past, the most popular manner of accomplishing automatic control of the timed events described above has involved the employment of electromechanical timing devices. The "events" in such devices were typically programmed by mechanical means such as bending tabs on synchronously rotating wheels inserting or removing pins in synchronously rotating drums or punching a tape with coded events and inserting the tape into the electromechanical timing mechanism. All of such programming techniques are characterized as comparatively complex and typically require a serviceman to be called in order to alter the program. Even the simple alteration occasioned by switching to or from daylight savings time typically requires the attendance of a serviceman. In addition, electromechanical systems which were capable of providing individual zone control and the like became relatively complex.
There have recently been introduced programmable devices for automatic control of electrical loads such as heating, air conditioning, lighting and alarm systems which can also be configured to ring bells in schools. Such systems are typically microprocessor based, and usually include a keyboard and display used to guide the event programming function. Such systems generally focus on an "event" as a relatively simple function affecting a single load, a single day, a single time on that day and a single switching operation. An example of such a system is described in U.S. Pat. No. 4,293,915 to Carpenter et al. In the school environment where there are often multiple zones on different schedules, differences in schedules from day to day, and the like, the number of events which need to be programmed into such systems expands rather dramatically. In addition, although the systems have keyboards for user programming, for a variety of reasons the programming task can become somewhat complicated, requiring a programmer with a comparatively high degree of skill and training.
One programmable clock which addresses the foregoing problems is the model 2424 manufactured by Rauland-Borg Corporation of Chicago, Ill. The model 2424 is disclosed in U.S. Pat. No. 4,387,420 to Singhi et al which is assigned to the assignees of the present invention. The user-programmable clock disclosed in the Singhi et al. patent employs a system which allows for the programming of comparatively complex events wherein each event includes a single time and any combination of days and zones. Programming of the clock disclosed in the Singhi et al. patent is simplified by providing an immediate indication of an erroneous entry along with a signal distinguishing between various potential types of errors.